1. Field of Invention
This invention relates to processes for applying coating compositions. More particularly, this invention relates to a process for applying a high solids coating composition using a high pressure spray system which has a low volatile organic solvent content.
2. Prior Art
The spraying of chemical coating compositions has been known for many years. Spray systems are of two basic types. The first type is the air-atomized spray system whereby air or another gas is used to atomize a coating into a fine mist which is then propelled onto the substrate to be coated. Air-based spray coatings suffer from well known deficiencies, including specifically that a high degree of overspray is often observed leading to a loss in the efficiency of use of the coating. In addition, in recent years increasing concerns have arisen about the large amounts of organic materials which are released into the atmosphere by air-atomized spraying of organic solvent containing coatings.
The second common type of spray process is the airless spray process whereby the coating itself is pressurized and this pressure is used to project the spray onto the substrate. Airless spray systems, in general, operate at relatively low pressures--no higher than about 1,600 to 2,000 psi. While airless spray applications reduce the amount of volatile organic material which is released into the atmosphere, they generally do not atomize the coating as well as conventional air-based sprays and therefore the resulting finish coating is often not up to the standards for smoothness exhibited by air atomized spray coatings.
These two common types of spray systems are often combined to obtain a system which contains some of the benefits of both. In the so-called "air-assisted air spray system" a small amount, less than about 30 psi, of air is introduced into a airless system to improve its overall application properties.
Prior to 1970, the overwhelming majority of industrially-applied coatings were based on organic solvents with solid contents less than 30 percent. Due to environmental regulations, use of such coatings has dramatically decreased. In recent years, because of the continued need to reduce the amount of volatile organic materials contained in coatings, work has continued on developing systems which will permit the spraying of relatively high solids, high viscosity coating compositions. One approach has involved the so-called "Unicarb process" which employs carbon dioxide as a super critical liquid under heat and pressure in combination with a more conventional solvent for the coating. A device embodying this concept is described in Vol. 2, High Solids Coating, March, 1987 entitled, "Super Critical Fluid Processing of Polymers Used in High Solid Paints and Coatings." Among the disadvantages of the Unicarb process, the foremost is its extremely high cost and its complex operating procedures: individual spray units often cost between $50,000 and $100,000 and require frequent and costly maintenance. This system is disclosed for example in U.S. Pat. Nos. 5,027,742 and 5,009,367.
An added impetus to the search for coatings which contain lower amounts of volatile organic solvents has been the recent changes in air pollution standards adopted in several states, including primarily California, and the recent regulatory amendments proposed by the United States Environmental Protection Agency ("EPA"). In each case, the effect of these regulations is to limit the amount of volatile organic materials which may be present in a coating. In addition, the proposed EPA regulations prohibit or limit the use of 189 chemicals, many of which are solvents commonly used in the coatings industry. For this reason, these chemical solvents will either have to be substantially reduced or eliminated in future coatings.
Heretofore, the most widely employed solution to problems posed by governmental regulation of the volatile organic solvent content of coatings has been to switch to water as the solvent for the coating. However, particularly for high quality wood finishes, water-based coatings have an adverse effect on wood grain termed "grain raising." Moreover, the use of aqueous based systems to control volatile organic emissions is particularly difficult where there is the possibility of the presence of high humidity levels during drying which will make water evaporation difficult.
Coating compositions for application to substrates, particularly wood substrates, may be either thermoset or thermoplastic but, preferably, are thermoplastic coatings, particularly of a lacquer type. The materials most widely used with wood substrates are cellulosics, such as nitrocellulose. Traditionally, nitrocellulose lacquer coatings for wood substrates are made using an alcohol wet nitrocellulose. However, when such alcohol wet nitrocellulose is applied using a high pressure, high solids coating process, air entrainment frequently results and, in addition, the flow characteristics of the coatings are sometimes imperfect. Difficulties also occur when this coating is "dry sprayed" or sprayed at higher temperatures.
High pressure spray pumps have been known for many years. Among the manufacturers of such pumps, Graco, Speedflo, Nordson, Ransberg, and Wagner are included. Examples of high pressure spray systems include those disclosed in U.S. Pat. No. 3,816,028.
Most spray coating systems typically operate at less than about 2,000 psi. See, for example, U.S. Pat. Nos. 3,433,415 and 3,726,711. Graco has offered pumps for sale which operate at pressures ranging from 2,700 psi to 4,050 psi. However, there has been no disclosure of the use of these pumps in conjunction with the coating compositions of the type described herein.
U.S. patent application Ser. No. 07/722,260 generally discloses a process for preparing high quality, high solids coatings compositions using a high pressure airless spray system. Disclosed therein as the preferred type of coating composition for use in the system was an alcohol wet nitrocellulose lacquer. The instant invention involves a significant improvement over that prior art system. Specifically, according to the instant invention, the alcohol wetting agent is replaced with a plasticizer as described hereafter. As a result, the coating compositions of the instant invention exhibit less air entrainment in the lacquer upon application, greater tolerance for variations in film thicknesses, improved dry sprayability, and a greater temperature range at which the composition may be utilized, thus allowing the use of higher viscosity coatings.
Accordingly, it is an object of this invention to prepare a high quality, high solids coating composition that can be applied using a high pressure airless spray coating system by the disclosed process.
It is another object of this invention to prepare a high quality coating composition for wood furniture, cabinetry and related substrates by the disclosed process.
It is a still further object of this invention to prepare a coating composition which may be applied at high viscosity with a low volatile organic content by the disclosed process and which upon application maintains the coating characteristics of prior art systems which use conventional technology.
It is yet another object of this invention to prepare a coating composition which may be readily sprayable at temperatures of up to at least 150.degree. F. and which may be applied by the disclosed process.
It is a still further object of this invention to prepare a coating composition which will meet the highest regulatory standards for volatile organic solvent amount and type and which may be applied by the disclosed process.
It is a still further object of this invention to prepare a coating composition comprised of a high solid nitrocellulose lacquer which can be applied by the disclosed process to a wood substrate to achieve high quality finishes.
It is a still further object of this invention to prepare a coating composition which is easy to apply using standard spray techniques.
It is a still further object of this invention to prepare a coating composition which may be applied by the disclosed process at a greater temperature range allowing the utilization of higher viscosity base coating products.
These and other objectives are obtained by preparing the coating compositions of the instant invention using the process described herein.